Blackberry Bushes are naturally occurring blocks that spawn Blackberries.
The Blackberry Bush is an easily transportable block. They infinitely spawn blackberries. You need to tap the bush in order for it to drop the Blackberry. The Blackberries can be easily seen on the full-size (grown) bushes, as they appear as dark red dots. Only harvest when you see the Blackberries on it.
When transplanted, they will need to grow to their full form to spawn Blackberries. Place them near light.
You can use bonemeal on the bush to make it grow through the stages faster as well as produce another bush on top once that bush is fully grown. Without bonemeal, the tallest the bush will grow is three blocks. Using bonemeal one can grow the bushes to any height desired.